Museum director Emily Ansenk, who had been in Istanbul, returned immediately to the Netherlands on Tuesday and gave an update to reporters attending a Tuesday afternoon press conference.

News of the heist "struck like a bomb," she told media. "It's every museum director's worst nightmare."

Ansenk declined to reveal any details about exactly how the theft of works from the private Triton Foundation collection took place. She described the Kunsthal's security system as "state of the art" and "functional."

'All involved want the public to still be able to see these kinds of special collections.'—Emily Ansenk, Kunsthal director

According to museum chair Willem van Hassel, the Kunsthal's security systems are automated and the facility does not use guards at the site. He added that police arrived on the scene five minutes after an alarm was triggered.

One of the most significant Dutch art thefts in years took place around 3 a.m. local time, police said.

"Those thieves got one hell of a haul," Chris Marinello, director of the international stolen art database The Art Loss Register, told The Associated Press.

Marinello said it was clear the robbers targeted some of the most valuable pieces in the collection. If sold legitimately at auction, the seven pieces could be worth "hundreds of millions of euros," he added.

However, now that the pieces have been registered as stolen, the thieves have limited options, including sale on the black market, or blackmailing the owners or the Kunsthal, which is a display space that has no permanent collection of its own (the name denotes "art gallery" in Dutch).

Gallery closed till Wednesday

Comprising work by more than 150 artists, the Triton Foundation collection of avant-garde art was assembled by multimillionaire businessman Willem Cordia and his wife, Marijke Cordia-Van der Laan.

The Cordia family's goal has been to put its collection — including work by top artists like Paul Cézanne, Marc Chagall, Salvador Dali, Edgar Degas and Andy Warhol — on display for the public. The collection was being exhibited publicly as a group for the first time as part of the Kunsthal's 20th-anniversary celebrations.

"We are shocked, but we will go on," Ansenk said, speaking on behalf of the family.

"All involved want the public to still be able to see these kinds of special collections and private collections."

The Kunsthal remained closed on Tuesday as police continued the investigation and reviewed videotaped footage. Officers have also called for any witnesses to come forward.

The museum will reopen on Wednesday.

A police officer investigates the surroundings of Rotterdam's Kunsthal art museum in the Netherlands on Tuesday, after an early-hours heist. The museum will reopen on Wednesday. ((Robin van Lonkhuijsen/Reuters))